I drove my car over here about 10 years ago. I went from Portsmouth to Bilbao on the cruise ferry "The Pride of Bilbao". I then drove from Bilboa to Cadiz and caught the ferry to Santa Cruz. With Ferry costs, Hotels and fuel, it cost me around 800 pounds, remember this was 10 years ago. The trip took 5 days. I also believe you can ship direct from Wales in a container, but not sure of costs.
A friend of mine had his car Right Hand Drive registered 6 months ago. The cost is based on the value of the car and paperwork costs. His total bill was around 800 Euros. Alternatively, if it is a custom or non previously registered car on the Island, you will have to a full techinal check done on the vehicle and it will have to be fully registered as a new type of vehicle. This can cost anything up to 8,000 Euros, depending on the age and value of the car.
Once registered over here, you are correct that the UK MOT and Road Tax does not have to be paid, as you will get a Spanish ITV and pay Spanish Road Tax.
At the end of the day you will be at a disadvantage driving a Right Hand Car over here. I can see no real point in bringing one over. If your car is fairly new, sell it and buy a new one here. Prices are aroung 20% cheeper here than the uk. If it is an older car, by the time you have got it over here and have paid all the bills, you could probably of bought a car of the same age, in better condition because of no weather damage, and not had to of gone through all the hassle.
Most people I know, who have brought cars over here, have regretted it later on, including myself.